Flash memory devices are classified into a NAND type and a NOR type depending on the construction and operation of a cell.
Also, flash memory devices are classified into floating gate series memory devices and Metal-Oxide-Nitride-Oxide-Semiconductor (MONOS) or Semiconductor-Oxide-Nitride-Oxide-Semiconductor (SONOS) memory devices depending on types of materials of a charge storage layer used in a unit cell.
The floating gate series memory devices are devices realizing memory characteristics using potential wells. The MONOS or SONOS series memory devices realize memory characteristics using a trap site within a bulk of a silicon nitride film, which is a dielectric film, or a trap site at an interface between dielectric films. The MONOS series memory device refers to memory devices whose control gates are of metal and the SONOS series memory devices refer to memory devices whose control gates are of polycrystalline silicon.
In particular, the SONOS or MONOS type flash memories have an advantage of relatively easy scaling, improved endurance, and regular threshold-voltage distribution compared to the floating gate type flash memories. However, the SONOS or MONOS type flash memories induce a degradation of characteristics of retention and endurance when a tunneling insulating film and a blocking insulating film are made thin in thickness for high integration.
As a solution to this drawback, it has been proposed that existing silicon oxide be substituted with high-k oxide as a blocking insulating film. However, traps generated at the time of depositing the high-k oxide for the blocking insulating film cause back tunneling of electrons through an electrode in an erase mode condition of high voltage. That is, there is a drawback that charges pass through the blocking insulating film from a control gate and remain in a bulk or an interface of a charge storage layer when a control voltage is applied to a control gate and Fowler-Nordheim (F-N) Tunneling is performed to move electrons from a nitride film that is a charge storage layer to a channel. Accordingly, the back tunneling deteriorates the characteristics of operation of flash memories to store information by a variation of a threshold voltage.
As a solution to the above drawback, the conventional art processes a blocking insulating film deposited by high temperature Rapid Thermal Anneal (RTA) of 800° C. or more to optimize the characteristics of the blocking insulating film. However, despite this solution, there still remains a limit in improving the characteristics of the blocking oxide film.